Ophthalmic lens



July 20 1926.

E. D. TILLYER OI HTHALMIC LENS Filed Nov. 6. 1924 INVENTOR IV I I v figl garDffillg cr. 55 4.

Patent July 20, 1926.

UNITED STATES PATENT OFFICE.

EDGAR ,"D. TILLYER, OF SOUTHIBRIDGE, MASSACHUSETTS, ASSIGNOR TO AMERICAN OP- TICAL COMPANY, OF SOUTHBRIDGE, MASSACHUSETTS, A VOLUNTARY ASSOCIA- TION OF MASSACHUSETTS.

OPHTHALMIC LENS.

Application filed November 6, 1924. Serial No. 748,180.

This invention relates to improvements in ophthalmic lenses, and has particular reference to bifocal lenses made of two pieces of glass, said lenses being commonly known in the art as fused bifocal lenses, and it also relates to an improved method or process of making the same.

This application is a continuation in part of my application Serial Number 631,506,.

filed April 12, 1923.

In general, a fused bifocal lenscomprises a major blank of glass of low index of refraction, having a spherical recessed surface in one side, and a segment of glass of high index of refraction fused into the recess and the surface of the blank on the segment side is provided with a spherical curved surface to give the desired added optical power of the segment section over the rest of the blank, the surface on the other side being finished to an optical surface to give the desired prescriptive power of the lens. Such lenses possess astigmatic aberration in the segment portion due to the obliquity of the vision through the segment, which astigma tism is especially pronounced in fused bifocal lenses due to the steep curves in the recessed portion, even when the distance portion is correct. r

The main object of the invention is to provide atwo part fused bifocal lens which shall be substantially free from astigmatism through the reading portion of the lens, and absolutely free from such astigmatism at a any one selected'point within said reading portion, if so desired.

A further object is to provide means for the elimination of such astigmatism in the reading portion by adding an excess corto rooting C1power ln one meridlan on the segmerit s1 and compensating on the opposite or prescription side for any error that the added correcting power may have intro-- duced into the distance portion of the lens.

Other objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, the preferred form of the invention befor several points,

. views.

In the manufacture 'of the improved lens, I start exactly the same as in the manufacture of .the ordinary fused bifocal lens,

namely taking a blank of low index glass, 1, finishing a countersunk recess 4 therein, and fusing a segment of highindex glass 5 in the recess 4, which roduces the blank from which the fused bi ocal lens is to be made. Hence I can start the manufacture of my lens with the ordinary commercial fused bifocal blank which is being manufactured and sold in lar e quantities at the present time.

Having obtained the fused blank, I next compute the astigmatic error through the segment portion for a lens of given prescription, said error being due to the oblique vision through the segment portion. These calculations are the well known prior art calculations of determining astigmatism of rays passing obliquely through. a lens, which may be found in standard text books for lens calculations.

'Let us assume the followin example: Prescription of plus 1 diopter sp ere, with a reading addition of plus 2.50 diopters. The oblique astigmatic error throughthe reading segment of this lens is about th of a diopter by the trigonometric computation and the average value taken. In order to remove this th diopter of astigmatism the easiest procedure is to first find out the effect of a' 1 diopter toric cylinder on the front surface of the distance field as measured in the distance field 45 might indicated by 6, because if we know the effect of one diopter as measured in the distance field 6 we can by direct proportion obtain the actual number of diopters as measured in the distance field necessary to eliminate one-tenth diopter error in the segment. We will assume the index of the glass for the distance portion to be 1.5232, and for the reading segment 1.6165, which are the in- 10 dices of the usual commercial article. It

.will be clear that if we put a 1 diopter toric cylinder curve on the major section 6, that it will have greater effect on the segment portion 7 because of the higher index of the glass of the segment portion, which will give the following ratio: a curve giving 1 iopter on the portion 6, will give 1.178 diopters on the segment portion 7, or an increase of 1.178 diopters less 1 diopter, which equals 0.178 diopter increase per dio ter on the segment portion over that of the istance portion.

From the foregoing it will be seen that we have increased the power of the distance portion 1 diopter, and'this must be compensated for on the other or prescription side of the lens by placing a minus 1 diopter toric cylinder curve on the prescription side. This will leave us 0.178 diopter increase 39 cylinder on the segment portion 7 for each diopter of increase on the front side of the distance rtion.

Now, t erefore, we have to remove 0.10 diopter astigmatism from the segment portion 7, which we do as follows: We find that for each diopter of cylinder value on the distance portion we can remove 0.178 dinpters cylinder from the segment portion, an we require to remove 0.10 diopter from the segment portion. Therefore, we will require 0.56 diopter toric cylinder on the seg ment face of the lens of opposite sign to the 0.10 diopter required to be removed from the segfnent with the same axis, or this e ex ressed in the transposed form by the usu methods of cylinder transposition, and likewise we will require a minus 0.56 diopter toric cylinder curve on the o posite or non-segment side. In other wor s,

the segment side must have a toric cylinder curve of 0.56 diopter on the low index portion, which will be a 0.66 diopter toric cylinder on the high index portion, and the opposite side of the lens must have a com- 5 pensating curve of opposite sign. If the with the compensating cylinder 0n the side opposite the segment side by the usual methods of combining oblique cylinders, as given 1n any standard text book.

It will be understood that by a toric curve or surface is meant a curved surface having one curvature in one major meridian, and another curvature in the other major meridian, but one of these curves may be zero. Where we refer to a 1 diopter toric cylinder is meant that the difierence in curvature in the two meridians is such as to produce an effect of 1 diopter in one meridian over the other.

The usual glass used in the production of these fused bifocals is what is commonly known as crown lass for the major portion 6, and flint glass for the segment portion 7.

It will be apparent, as indicated in Figure 1, that the shape of the segment 7, in plan View, will not e exactly round or circular, but will be slightly elliptical, the ellipse bemg elongated on the meridian having the weaker power, and shortened on the meridian having the stronger power.

Reference has been made to the vision through the segment portion being oblique. This is indicated in Figures 2 and 4. It will be noted thatthe segment or reading portion lies below the axial line 8, the center of the eye 9 being located on the axial line 8 at about 27 millimeters from the rear surface of the lens. It is clear that the line of vision 10 is in a descending oblique position and passes obliquely through the segment or reading portion and not normally there- 'through, also obliquely through that portion of the major blank lying behind the segment, and the correction that we are referring to is the sum of all of these variations as distinguished from going through the distance portion only at a corresponding angle. It will be noted in Figure 4, where the segment is on the eye side. of the lens, that the conditions are much more severe than those in Figure 2, where the segment-is on the outside surface of the lens: in Figure 4 the line of direction will be more oblique than in Figure 2, particularly at the upper part of the segment7. The eye is indicated by the dotted line 11 in Figures 2 and 4.

In Figure 3 is shown a section through the lens in the axial line in the horizontal meridian, taken on line 33 of Figure 1. It will be clear that this section is entirely through the distance portion of the lens and does not touch the reading portion.

From the foregoing description it will be seen that I have provided means of compeneating the astigmatic error in the segment portion by adding a toric curve to one side of the lens and compensating for the added toric in the distance portion by putting a toriccurve of opposite sign on the opposite side of the lens, and in this way the astigmatic error in the reading portion of any desired prescription may be determined and compensated for.

It is well known that the astigmatism m "the ordinary usable powers of fused bifocal lenses is rather high; as for instance in the example we have given there is approximately th dioptererror in a rather weak prescription, that is, a 1 spherical power with a 2.50 reading addition. This error in creases rapidly with the increased power of the prescription.

It will be understood that in the manufacture of this bifocal no new machinery or tools are needed. As stated, the original blank is the prior art blank for the usual fused bifocal lens, and the toric cylinder curves are ground with the ordinary toric cylinder tools, which have been in use in the art for many years.

I do not limit myself to the exact details illustrated and described herein, but reserve the right to make such changes as fall within the scope of the claims without departing from the spirit of the invention.

. ing a spherical recess therein,

ce h I claim:

1. An ophthalmic lens, comprising a dis tance portion of one index of refraction hava segment reading portion of different index of refraction secured in the recess, and a lens surface on the segment side of the lens, the difference of whose astigmatic surface powers on the distance portion and the segment portion is sufiicient to overcome the astigmatic aberration of the segment of the rea ing portion, and an optical surface on the other side of the lens that will ive the required prescription power and a so compensate the distance portion for the change in power that has been made in it to correct the segment portion.

An ophthalmiclens, comprising'a distance portion and a reading portion of differ-ent indices of refraction having an optical surface ground on one side with such a difference of astigmatic power between the reading portion and the distance portion that it will overcome the astigmatic aberration in the reading portion, and an optical surface on the other side having an addition in power sufficient to compensate the distance portion for the addition in power that has been made to correct the astigmatism of the reading portion, and also the required prescription power.

3. An ophthalmic lens, comprising a blank aving a spherical recess in one side, a segment reading portion inserted in said re- --cess, said segment being of a different refractive index, one side of the blank being ground to a toric surface to overcome the astigmatic aberration in the segment, and

the other side being finished to an optical surface having an addition in power sufli cient to compensate the addition in power of the non-segment portion to overcome the astigmatic aberration in the segment portion, and also to give the required prescription power.

4. An ophthalmic lens comprising a distance portion of low index of refraction having a spherical recess therein, a segment reading portion of high index of refraction secured in the recess, and a. lens surface on the segment side of the lens, the difference of whose asti matic powers on the distance portion and t e se ent portion is sufiicient to overcome the o lique astigmatic aberration of the reading portion, and an optical surface on the other side of the lens that will give the required prescri and will compensate for the a ditional astigmatism introduced into the distance field in correctin the reading field.

5. An op thalmic lens of the character described, comprising a distance portion of crown glass having a spherical recess on one side below its center, a reading portion formed by a flint glass segment inserted in said recess, the segment side of the lens bein ground in toric form, the difference of wliose astigmatic powers on the distance portion and the segment portion is suiiicient to overcome the astigmatic aber ration in the readin portion, and the op-. posite side of the i an optical surface to give the required prescription ower and having an addition in power su cient to neutralize the change in power to the distance portion caused by correction of the reading portion.

6. In an ophthalmic lens of the character escribed, a blank of crown lass forming a distance portion having a sp erical recess in one side below its center, a segment of flint glass inserted in said recess and forming a reading portion, the segment side of the lens being finished in toric form to overcome the astigmatism in the reading or tion, and the other side being finishe in toric form of opposite sign suflicient in power to give the required prescription power and to neutralize the addition given to the distance portion to overcome the astigmatism in the reading portion.

7. An ophthalmic lens blank comprising a distance portion of one index of refraction having a spherical recess therein, a ment reading portion of difi'erent index of refraction secured in the recess, and a toric lens surface on the'segment side of the blank having an excess of power in one major meridian over the power of the other ma'or meridian equal to the of the segment reading portion and positioned to neutralize the said astigmatic aberration of the segment reading portion.

tion powerns being finished to segastigmatic' aberration 8. An ophthalmic lens blank comprising a distance portion of one index of refraction having a spherical recess therein, a segment reading portion of different index of refrac- 5 tion secured in the recess, and a lens surface on the segment side of the blank, the difference of whose surface powers on the distance portion in one major meridian over the other major meridian and the segment portion is equal to the astigmiltic aberration of the segment reading portion, and positioned to neutralize the said astigrnntic aberration of the segment reading portion.

EDGAR D. TILLYER. 

